A. Field of the Invention
The present invention relates to a transmission mechanism and method for an image scanner, especially to an image scanner having a moveable document sheet table and a moveable image reading device for being driven in opposite directions at the time when capturing the image of a document sheet, thereby to increase the speed of scanning.
B. Description of the Prior Art
Conventional image scanners have been designed with various transmission mechanisms for the concern of image quality control, including the single-carriage transmission mechanism, the double-carriage transmission mechanism, or the paper-driven transmission mechanism. In general, these transmission mechanisms are driven by step motors for reading the image information of an original linearly and step by step. Refer to FIG. 1 for showing an example of a conventional single-carriage transmission mechanism for a flatbed scanner. The sheet table 12 is fixed on top of the housing for placing a document sheet. An image reading device 13 is driven along the sheet table 12 by a transmission device 11. Thus, the image information of the document sheet can be read progressively while the image reading device 13 is driven along the sheet table 12 by the step motor.
Refer to FIG. 2 for showing the image scanners of the paper-driven type. The image reading device 21 is fixed in the housing 22. The paper feeder (not shown) can drive the document sheet 23 progressively through top of the image reading device 21 for allowing the image reading device 21 to capture the image information therefrom. The similarity between these two types of transmission mechanisms is that only one element is moving at a time. Since the transmission mechanisms of these scanners are controlled by step motors, therefore the scanning speed is limited by the speed of the step motors.
To increase the scanning speed, an approach is using a high-speed step motor to increase the transmission speed and thus reduce the scanning time. However, the speed of the existing step motor is still slower than the speed of the image reading. So, up scaling the speed of a step motor can improve very little on the scanning speed.
Accordingly, it is an object of the present invention to provide a new transmission mechanism which can efficiently increase the scanning speed by providing a bidirectional transmission mechanism to drive a sheet table and an image reading device in opposite directions and at the same time.
It is another object of the present invention to improve the scanning efficiency based on the well-developed step motor technology and transmission mechanism.
In accordance with the present invention, the present invention provides a bidirectional transmission mechanism for an image scanner. The bi-directional transmission mechanism provides a first transmission device for driving a sheet table in a first direction, and a second transmission device disposed below the sheet table for driving an image reading device in a second direction. The two transmission devices are moving towards opposite directions and at the same time when the scanning operation is enabled. Since these two transmission devices are moving towards each other, the scanning speed will be at least doubled if they are driven by step motors of the same speed.
The present invention mainly encompasses a sheet table for placing a document sheet. A first transmission device is provided for supporting and connecting to the sheet table for driving the sheet table from a first direction to a second direction. An image reading device disposed under the sheet table reads the image information of the document sheet. And a second transmission device disposed in parallel to the first transmission device is provided for supporting and driving the image reading device from the second direction to the first direction at the same time when the scanning operation is enabled.